crystallinity in polymers sheaf-like arrangement of lamellae in a blend of polyethylenes system:...

Crystallinity in Polymers

Sheaf-like arrangement of lamellae in a blend of polyethylenesSystem: Polyethylene (PE), Composition: LPE:BPE 3:1

An image of an alkane crystal taken by AFMSystem: Alkane, Composition: C36H74

An image of a single crystal alkaneSystem: Alkane, Composition: C294H590

Single PE spherulite AFM

Maltese cross spherulites

Thermodynamics of melting and crystallization: First order transitions

Amorphous v Crystalline Polymers Thermo-mechanical properties

Density Increase

Pro

pert

y

Shrinkage, Stiffness, Tensile strength, Hardness, Heat deflection, Chemical resistance

Weatherability

Impact strength, Ductility

Low density polyethylene (LDPE) 915-929 45-65Medium density polyethylene (MDPE) 930-939 65-75High density polyethylene (HDPE) 940-965 75-90

Material Density (kg/m3) % Crystallinity

Thermal Transition Points of Select Polymers

Rule of Thumb for Tg’s and Tm’s

For symmetrical polymers: Tg = 0.5 Tm (Kelvin)

For asymmetrical polymers: Tg = 0.66 Tm (Kelvin)

Polyvinyl chloride Tg = 81 + 273 = 377 K

Tm = Tg/0.66 = 354/0.66 = 536 K or 263°C

Experimentally Tm = 273 °C

Polyvinylidene chloride Tg = -18 + 273 = 255 K

Tm = Tg/0.50 = 255/0.5 = 510 K or 237°C

Experimentally Tm = 200 °C

Rule of Thumb for Tg’s and Tm’s

Caution: Its just a rule of thumb:

Atactic polystyrene Tg = 104 + 273 = 377 K

Tm = Tg/0.66 = 377/0.66 = 571 K or 298 °C

Experimentally Tm = 523 K or 250 °C

Crystalline Polymers (really semicrystalline)

Polar functionality

Thermodynamic of Crystallization

For melting Sf is positive

Intramolecular interactions (Hf) favor crystallization & higher Tm

Hydrogen bonding20 kJ/mol

Van der Waals: 2 kJ/mole

Explain why Nylon 6 has a lower Tm than Kevlar

Entropic Contributions to Tm

Flexible Chains have numerous conformations

Nylon 6

Rigid Chains have fewer conformations

Kevlar example

Polymer symmetry and Melting Point

Molecular Weight Influence on Tm

• Melting temperatures of n-alkanes (up to C100) as a function of chain length.

Methods for Inducing Crystallization in Polymers

• Slow cooling of molten polymer

• Annealing between Tg and Tm

• Evaporation of solvent

• Shear & disintanglement

• Stretching and alignment of macromolecules

Characterization of Crystalline Polymers: Diffraction

Rare to get single crystals: Powder XRD or films

Polyethylene’s Orthorhombic Unit cell

Vinyl Polymer Crystals: Substituents favor helical conformation

Characterization of Crystallinity in Polymers

Polymers generally have crystalline and amorphous contributions

Lamellar Structure of Polymer crystals

Polymer single crystals: Graduate students nightmare

Still lamellar structures

Validation of Models

Dislocations in Polymer Crystals

From singhle crystals to Aggregate structures

Polyethylene Spherulites

Spherulite Growth from Lamellar crystals

TEM of spherulite structure in natural rubber(x30,000).• Chain-folded lamellar crystallites (white lines) ~10nm thick extend radially.

Crystalline structures in polymers

crystallineregionamorphousregion

• % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases.

0unload/reload0brittle failureplastic failure204060246σ( )MPaεxx -semicrystalline

case

amorphousregionselongate

crystalline regions align

crystallineregions

slide

8 onset ofnecking

,aligned-cross

linkedcase

networkedcase

Initial Near Failurenearfailure

Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along plastic response curve (purple) adapted from Fig. 15.12, Callister 6e.

Tensile Response: Brittle & Plastic

TemperatureTg Tg

EHigher % S-Cryst

Cooling rates for semi-crystallines are important!

Amorphous polymer properties do not depend on cooling rate.

Amorphous

Semicrystalline polymer properties depend on final degree of crystallinity, and hence the rate of cooling.

Lower % S-Cryst

Achieved using slower cooling rates.

Micrographs of Polymer Spherultes

Seeing Maltese Crosses: Polarizing Microscopy

Polarizing Optical Microscopy

Formation of Ring Pattern: Lamellar Twisting

Microfibriallar Morphology

Polyethylene Fibers Nucleated on Si-C fibers: Shish-Kebobs

Branching on Crystallinity

Which one will be more likely to crystallize?

Linear crystallizes easier (HDPE = linear; LDPE = branched)

Nucleation Rates between Tg and Tm

Primary Crystallization

Cry

sta

llin

ity (

%)

Cooling rate (oC/s)

10

20

30

40

00.01 0.1 1.0 10 100

Slow Cooling

Quenchin

g

Early stages of crystallation of PEEK in the presence of a carbon fibre.

Effects of Crystallinity

1) Strength: Stronger & Stiffer

2) Optical: Opaque (scattering by spherulites)

3) Higher density

4) Less Soluble

5) Less Permeable

Smaller interchain distancesStronger intermolecular forces